Along with the recent development of mobile communication technologies, the need for a miniature and lightweight surface acoustic wave device mounted module with improved electrical characteristics, which is used as an interstage reception filter, an interstage transmission filter and an antenna filter for mobile communication equipment, has been increasing.
A conventional surface acoustic wave (SAW) device mounted module, for example, is described in IEEE Transactions on Vehicular Technology, vol. 38, No. 1 (1989), pp.2-8. This conventional SAW device mounted module has a surface acoustic wave element mounted on a can-like package. An aluminum wire provides continuity across the input-output terminals of the package and the surface acoustic wave element. Or, instead of a can-like package with protruding input-output terminals, a package which can be mounted on plates such as a ceramic package has recently been requested, particularly for mobile communication equipment. For this type of package and a can-like package, however, a bonding wire is required to establish continuity with surface acoustic wave element. Therefore, the miniaturization of a SAW device mounted module is limited when one of these packages is used for the SAW device mounted module, since the height of the bonding wire is limited and a land for the bonding wire is required on the package.
A face down bonding (or flip chip bonding) is used as a method of miniaturizing a SAW device mounted module. In this method, the surface acoustic wave element faces a circuit substrate and conductive bumps are used to establish continuity across the element and the substrate. In other words, a bonding wire is not required in this method.
Use of the face down bonding method to a surface acoustic wave element is mentioned in Proceedings on 1984 Ultrasonics Symposium (1984), pp. 36-39. However, when this method is applied to a mobile communication equipment, the equipment cannot be miniaturized or kept in an airtight condition. Another applied example of the face down bonding method is mentioned in Proceedings on IMC Symposium (1992), pp. 99-103. The problems typical to a surface acoustic wave element make it difficult to apply the face down bonding method to a SAW device mounted module. For example, a surface acoustic wave element applied for mobile communication equipment, in particular, uses a piezoelectric substrate with high pyroelectricity, so that when a frequency band (pass band frequency for a surface acoustic wave element used as a band pass filter) is high, the line width of an inter-digital transducer (IDT) used for the element ranges from 0.5 .mu.m to 1 .mu.m. Therefore, when the temperature changes by processes of forming conductive bumps, the IDT is likely to be broken by the pyroelectricity of the substrate. In addition, a space around the surface acoustic wave element is required for smooth propagation of surface acoustic waves along the functional surface of the element.
In other methods of miniaturizing mobile communication equipment, a surface acoustic wave element is combined with a matching circuit or integrated with other parts. The combination of the element with the matching circuit is particularly required when the element is used as an IF (intermediate frequency) filter for mobile communication equipment. If the IF filter is applied as a surface acoustic wave element, input-output impedances generally become high so that an impedance matching circuit is required to connect the element with an outside circuit. An example of an integrated circuit including a surface acoustic wave element (IF section) is mentioned in Proceedings on Ultrasonics Symposium (1986), pp. 283-288. In this example, packaged devices are integrated with each other into one body so that this method is not effective in miniaturizing mobile communication equipment.
A conventional SAW device mounted module disclosed in Published Unexamined Japanese Patent Application No. Hei 5-291864 is manufactured by the steps of:
forming gold or aluminum bumps on electrode pads of a surface acoustic wave element; PA1 transfer-coating a conductive resin on top of the bumps; PA1 placing the bumps on the surface of a ceramic substrate formed with a wiring pattern; PA1 heating and curing the conductive resin to bond the element to the ceramic substrate; and PA1 adding an insulating resin around the element to increase the adhesion between the element and the substrate.
In this conventional SAW device mounted module, a space is provided around the IDT of the SAW element, and the viscosity of insulating resin is arranged to prevent adhesion of the resin to the IDT. A metal lid is attached to the substrate by an insulating resin and the SAW element is kept in an airtight condition. As another advantage of this SAW device mounted module, IDT is not broken during the manufacturing processes since the element is not exposed to a high temperature in this method.
This conventional SAW device mounted module, however, includes a monolayer ceramic substrate, and only a two-dimensional wiring is used with this SAW device mounted module. Therefore, the size of the SAW device mounted module is large when an impedance matching circuit is formed on the ceramic substrate or when a module is formed by integrating the surface acoustic wave element with other parts. Since a wiring pattern is formed on the surface of the ceramic substrate, a metal lid cannot be sealed by a conductive material, so the electromagnetic shield of this conventional SAW device mounted module becomes insignificant.